A common method of increasing productivity of a hydrocarbon-bearing formation penetrated by a wellbore is to subject the formation to stimulation techniques, such as hydraulic fracturing. Fracturing of a subterranean formation is accomplished by pumping a fracturing fluid into the wellbore which penetrates the formation at a sufficient pressure and flow rate such that cracks or fissures are opened into the surrounding formation. The fracturing fluid typically contains a proppant which functions to prop open created fractures such that the fracture provides larger flow channels through which an increased quantity of a hydrocarbon may flow. Productive capability of the well is therefore increased.
Fracturing fluids are typically viscous gels in order to provide a medium that will adequately suspend and transport the solid proppant as well as impair the loss of fracture fluid to the formation during the treatment. The fracturing fluid, typically water or oil-based, usually contains a viscosifying polymer which serves to increase the viscosity of the fluid. The fracturing fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that more viscous fluids will produce longer and wider fractures.
Common viscosifying agents for use in fracturing fluids are polysaccharides (galactomannans), such as guar gum and guar gum derivatives. The most commonly used guar gum derivatives are carboxymethyl guar (CMG) and carboxymethyl hydroxypropyl guar (CMHPG). Gelled fracturing fluids containing guar gum derivatives are highly desirable for certain fracturing applications because they exhibit excellent proppant transport properties and high thermal stability. As such, they have particular applicability in high temperature applications.
Further enhancement of the viscosity of the fracturing fluid may be obtained by using a crosslinking agent in conjunction with the viscosifying polymer. Typical crosslinking agents include those which are titanium, aluminum, boron and zirconium based as well as those which are boron based.
Typically, carboxylated guars used in hydraulic fracturing operations are formed by derivatizing the guar seed endosperm, often referred to as a “split”, which is generally semi-spherical in shape (about ⅛″ long and 1/16″ in diameter) and from about 0.5 mm3 to about 1 mm3 in volume. Guar splits are typically treated with aqueous caustic to cause swelling, and then exposed to a derivatizing agent, such as sodium chloroacetate (“SCA”) in an amount necessary to provide a desired molar degree of substitution (“DS”) value, which is the number of hydroxyl groups on each pyrannoside sugar occupied by the derivative. Typical guar split derivatization yields using SCA are about 60%, with about 40% of the SCA being consumed to form undesirable byproducts such as glycolic acid. Following derivatization, the splits are typically cooled and washed to remove excess caustic and unwanted byproducts before being dried and ground to a powder.
After the fracturing fluid is injected into the formation to produce the fracture, the viscosity of the fluid is reduced as the gel slowly degrades. As the viscosity decreases and pressure is released, the fractures relax and close on the proppant pack. The well is then cleaned up by flowing the fluid of decreased viscosity out of the well. Clean-up may be hindered by formation damage caused by gel residue left in the proppant pack. The effectiveness of well clean-up and the degree of gel residue left in the fracture often determine ultimate hydrocarbon production from the well. It is desirable to reduce loading of the viscosifying polymer in the fracturing fluid as much as possible in order to reduce formation damage. In addition, when equivalent fracturing fluid performance may be obtained with lower loadings of polymer, overall treatment costs are reduced.
A need exists for improved fracturing fluids which exhibit high viscosity at lower polymer loadings. It is desirable that such fluids further cause minimal formation damage such that clean-up may be more efficient, thereby improving production of hydrocarbons from the well.